1. Field of the Invention
The present invention relates to dendrite or asteroidal titanium dioxide micro-particles, a process for producing the same, electroconductive dendrite or asteroidal titanium dioxide micro-particles comprising said titanium dioxide micro-particles as substrate which have been subjected to an electroconductivity imparting treatment, and a process for producing the same. The titanium dioxide micro-particles according to the present invention are useful for sunscreen preparations or cosmetics, UV screening paints, UV screening or shielding materials in plastics, electroconductivity imparting or antistatic materials to be used in plastics, paints, rubbers and fibers, as well as electroconductivity imparting agents or substrates to be used in supports for recording materials such as electrophotographic copying paper and electrostatographic recording paper.
2. Description of the Related Art
Fine titanium dioxide particles having a primary particle size of 0.1 .mu.m or less are transparent because they transmit rays of visible light when incorporated in resin films or shapes, and on the other hand capable of shielding ultraviolet radiation to protect materials which may be discolored or degenerated with ultraviolet radiation. Thus they have different properties from those of pigment titanium dioxide having a primary particle size of about 0.15 to 0.5 .mu.m so that they have been utilized in sunscreen preparations, UV screening paints, and UV screening or shielding materials for plastics.
With respect to the transparency and UV shielding property of the aforementioned titanium dioxide micro-particles, the former property increases inversely proportional to the particle size, while the latter varies with the particle size with reaching a maximum in a specific range of the particle size and then decreasing as the particle size increases over the specific range. That is, the UV shielding property is reduced at particle sizes either smaller or larger than the specific range. Therefore, development of titanium dioxide micro-particles having simultaneously satisfactory transparency and UV shielding property has been desired.
The aforementioned titanium oxide micro-particles having a average primary particle size of about 0.05 to 0.1 .mu.m have been produced by, for example, a process comprising firing hydrated titanium dioxides produced by hydrolyzing a solution of titanyl sulfate or titanium tetrachloride, or a titania sol obtained by thermally treating the hydrated titanium dioxides in the presence of hydrochloric acid after treated with a caustic alkali. However, sintering of particles are highly apt to occur in the course of the firing process, so that it is difficult to achieve substantial dispersion of the primary particles in various media for numerous applications resulting in unsatisfactory UV shielding effect which is vigorously desired to be improved.
Titanium oxide compounds which have been proposed as electroconductivity imparting materials include (1) spherical electroconductive titanium oxide particles which substrates are spherical titanium oxide particles or fine spherical titanium oxide particles, (2) electroconductive materials comprising primarily fibrous potassium titanate, and (3) acicular electroconductive titanium oxide particles based on high quality acicular titanium oxide particles having a length of 1 to 10 .mu.m.
Generally speaking, electrical conductivity-impartig materials of an acicular form (including fibrous form), as compared with spherical form, and furthermore the materials which are lower in their powder resistivity, namely, higher in electrical conductivity, can give resin articles and rubber articles of the desired conductivity even with the addition thereof in a small amount to the article. The spherical electroconductive titanium oxide particles referred to in the above item (1) are generally required to be incorporated in a considerably large amount into media such as resins and rubbers to impart desired electroconductivities to the media due to spherical particles, though their powder bodies have a very excellent electroconductivity, for example, a resistivity of 1 to 10 .OMEGA. cm. For this reason, the use of spherical titanium oxide particles produces problems of reducing the strength of the products such as electroconductive resins and rubbers and of being uneconomical. The electroconductive materials referred to in the above item (2) have no problem in configuration, but disadvantages that the powder body of the materials has a high resistivity and the fibrous materials are liable to break during dispersing. Acicular electroconductive titanium oxide particles referred to in the above item (3) overcome the problems described above with respect to (1) and (2) in that only a small amount of the acicular particles is required to be added to achieve a very excellent electroconductivity, but they cause problems of flatness of the surfaces of coatings obtained by spreading a paint composition containing the acicular particles and a medium on a substrate, because they are too long in addition to being acicular. There is a need for titanium oxide particles which are useful for an antistatic agent for magnetic recording media, more excellent in transparency and surface flatness of magnetic layers as well as in electroconductivity imparting effect according to a demand for increasing recording density.